The influence of tectonic setting on the hydrological characteristics of deeply weathered terrains: evidence from Uganda

Deeply weathered landscapes occur throughout equatorial regions and are transformed by cycles of deep weathering and stripping. These processes are driven by rainfall that infiltrates the subsurface as groundwater recharge or runs across the landsurface. Recent work in Uganda demonstrates that long-term (10(5)-10(6) a) cycles of deep weathering and stripping are tectonically controlled. The influence of tectonics on the present-day hydrological regime and hence, contemporary geomorphic process, is examined in this study. The hydrological characteristics of two humid basins in Uganda were determined using soil-moisture balance techniques, hydrograph separation methods, borehole hydrographs and stable isotope ((2)H, (18)O) tracers. These were, in turn, related to the dominant geomorphic process (deep weathering or stripping) suggested by the local tectonic history. In central Uganda, where deep weathering of the landsurface has occurred since the Miocene, soil-moisture balance calculations, monitoring-well records and stable isotope data show that groundwater recharge is derived from monsoon rainfall, is in the order of 120 mm a(-1) and greatly exceeds surface runoff of 3-4 mm a(-1). Basin output occurs as evapotranspiration from a wetland that runs centrally through the catchment. Consistency in this discharge, despite highly seasonal rainfall, reflects a large basin storage that is permitted by a thick weathered mantle. In south-western Uganda, where stripping of the landsurface has occurred since the mid-Pleistocene, soil-moisture balance calculations and hydrograph separation techniques indicate ground-water recharge is restricted to years of exceptionally high rainfall. Annual surface runoff (34 mm a(-1)) generated primarily from monsoon rainfall events, exceeds the recharge flux (8-15 mm a(-1)). Streamflow discharge varies considerably (from 0.2 to > 14 m(3) s(-1)) and occurs along well-incised drainage channels. The comparatively smaller basin storage results from a thinner, partially stripped weathered mantle. The influence of tectonics is indicated by the conclusion that deep weathering, associated with prolonged tectonic stability in central Uganda, is facilitated by a recharge-dominated hydrological regime whereas stripping, prompted by mid-Pleistocene tectonic uplift in south-western Uganda, continues to occur under a runoff-dominated regime.